Back in the summer I’d made the mistake of telling my mum what I did for a living. Not the police bit, which of course she already knew about having been at my graduation from Hendon, but the stuff about me working for the branch of the Met that dealt with the supernatural. My mum translated this in her head to ‘witchfinder’, which was good because my mum, like most West Africans, considered witchfinding a more respectable profession than policeman. Struck by an unanticipated burst of maternal pride she proceeded to outline my new career path to her friends and relatives, a body I estimate to comprise at least twenty per cent of the expatriate Sierra Leonean community currently resident in the UK. This included Alfred Kamara who lived on the same estate as my mum and through him his thirteen-year-old daughter Abigail. Who decided, on the last Sunday before Christmas, that she wanted me to go look at this ghost she’d found. She got my attention by pestering my mum to the point where she gave in and rang me on my mobile.

I wasn’t best pleased because Sunday is one of the few days I don’t have morning practice on the firing range and I was planning a nice lie-in followed by football in the pub.

‘So where’s this ghost?’ I asked when Abigail opened her front door.

‘How come there’s two of you?’ asked Abigail. She was a short skinny mixed-race girl with light skin that had gone winter sallow.

‘This is my colleague Lesley May,’ I said.

Abigail stared suspiciously at Lesley. ‘Why are you wearing a mask?’ she asked.

‘Because my face fell off,’ said Lesley.

Abigail considered this for a moment and then nodded. ‘Okay,’ she said.

‘So where is it?’ I asked.

‘It’s a he,’ said Abigail. ‘He’s up at the school.’

‘Come on then,’ I said.

‘What, now?’ she said. ‘But it’s freezing.’

‘We know,’ I said. It was one of those dull grey winter days with the sort of sinister cold wind that keeps on finding ways through the gaps in your clothes. ‘You coming or not?’

She gave me the patented stare of the belligerent thirteen-year-old but I wasn’t her mother or a teacher. I didn’t want her to do something, I wanted to go home and watch the football.

‘Suit yourself,’ I said and turned away.

‘Wait up,’ she said. ‘I’m coming.’

I turned back in time for the door to be slammed in my face.

‘She didn’t invite us in,’ said Lesley. Not being invited in is one of the boxes on the ‘suspicious behaviour’ bingo form that every copper carries around in their head along with ‘stupidly overpowerful dog’ and being too fast to supply an alibi. Fill all the boxes and you too could win an all-expenses-paid visit to your local police station.

‘It’s Sunday morning,’ I said. ‘Her dad’s probably still in bed.’

We decided to wait for Abigail downstairs in the car where we passed the time by rooting through the various stake-out supply bags that had accumulated over the year. We found a whole tube of fruit pastels and Lesley had just made me look away so she could lift her mask to eat one when Abigail tapped on the window.

Abigail, like me, had inherited her hair from the ‘wrong’ parent but, being a boy, mine just got shaved down to fuzz while Abigail’s dad used to troop her over to a succession of hair salons, relatives and enthusiastic neighbours in an attempt to get it under control. Right from the start Abigail used to moan and fidget as her hair was relaxed or braided or thermally reconditioned but her dad was determined that his child wasn’t going to embarrass him in public. That all stopped when Abigail turned eleven and calmly announced that she had ChildLine on speed-dial and the next person who came near her with a hair extension, chemical straightener or, god forbid, a hot comb was going to end up explaining their actions to Social Services. Since then she wore her growing afro pulled into a puffball at the back of her head. It was too big to fit into the hood of her pink winter jacket so she wore an outsized Rasta cap that made her look like a racist stereotype from the 1970s. My mum says that Abigail’s hair is a shameful scandal but I couldn’t help noticing that her hat was keeping the drizzle off her face.

‘What happened to the Jag?’ asked Abigail when I let her in the back.

My governor had a proper Mark 2 Jaguar with a straight line 3.8 litre engine that had, because I’d parked it up in the estate on occasion, passed into local folklore. A vintage Jag like that was considered cool even by 3G kids while the bright orange Focus ST I was currently driving was just another Ford Asbo amongst many.

‘He’s been banned,’ said Lesley. ‘Until he passes the advanced driver’s course.’

‘Is that because you crashed that ambulance into the river?’ asked Abigail.

‘I didn’t crash it into the river,’ I said. I pulled the Asbo out onto Leighton Road and turned the subject back to the ghost. ‘Whereabouts in the school is it?’

‘It’s not in the school,’ she said. ‘It’s under it – where the train tracks are. And it’s a he.’

The school she was talking about was the local comprehensive, Acland Burghley, where countless generations of the Peckwater Estate had been educated, including me and Abigail. Or, as Nightingale insists it should be, Abigail and I. I say countless but actually it had been built in the late Sixties so it couldn’t have been more than four generations, tops.

Sited a third of the way up Dartmouth Park Hill, it had obviously been designed by a keen admirer of Albert Speer, particularly his later work on the monumental fortifications of the Atlantic Wall. The school, with its three towers and thick concrete walls, could have easily dominated the strategic five-way junction of Tufnell Park and prevented any flying column of Islington light infantry from advancing up the main road.

I found a parking space on Ingestre Road at the back of the school grounds and we crunched our way to the footbridge that crossed the railway tracks behind the school.

There were two sets of double tracks, the ones on the south side sunk into a cutting at least two metres lower than those to the north. This meant the old footbridge had two separate flights of slippery steps to navigate before we could look through the chain link.

The school playground and gym had been built on a concrete platform that bridged the two sets of tracks. From the footbridge, and in keeping with the overall design scheme, they looked almost exactly like the entrance to a pair of U-boat pens.

‘Down there,’ said Abigail and pointed to the left-hand tunnel.

‘You went down on the tracks?’ asked Lesley.

‘I was careful,’ said Abigail.

Lesley wasn’t happy and neither was I. Railways are lethal. Sixty people a year step out onto the tracks and get themselves killed – the only upside being that when this happens they become the property of the British Transport Police, and not my problem.

Before doing something really stupid, such as walking out onto a railway track, your well trained police officer is required to make a risk assessment. Proper procedure would have been to call up the BTP and have them send a safety qualified search team who might, or might not, shut the line as a further precaution to allow me and Abigail to go looking for a ghost. The downside of not calling the BTP would be that, should anything happen to Abigail, it would effectively be the end of my career and probably, because her father was an old-fashioned West African patriarch, my life as well.

The downside of calling them would be explaining what I was looking for, and having them laugh at me. Like young men from the dawn of time I decided to choose the risk of death over certain humiliation.